Hot metal surfaces, as found in chambers used to perform ionization in mass spectrometers, are known to interact with certain chemical species to decompose said species before ionization takes place. For instance, chlorinated compounds, such as the pesticides Lindane and Methoxychlor, will decompose in such a way that the mass spectrum produced following ionization will be quite different from that of a standard sample, run on an instrument with a chamber having deactivated surfaces. Other classes of compounds, such as the pesticide Parathion, will also show decomposition before ionization, with the resulting spectrum being of a different substance. These chemical degradations are of particular importance when performing quantitative analysis of low levels of materials, such as is done in gas chromatograph/mass spectrometer (GC/MS) analysis of pesticide residues, drug residues and metabolites, or trace analysis of organic compounds.
In addition to sample degradation, active metal surfaces will also influence chromatographic peak shape. Materials eluting from a gas chromatographic column impinging on a surface may stick to the surface in such a way that the shape of a symmetrical peak is changed to one with a long tail.
Similar sample degradation problems have also been found in heated sample injectors, metal transfer lines and detectors of gas chromatographs.
One way of minimizing these effects has been to treat the metal surfaces with some form of passivating agent to cover up or destroy active surface sites. An example is the use of alkylchlorosilanes and similar types of silynizing agents, to treat injectors, chromatographic columns, transfer lines, and detectors in gas chromatography.
Such treatments of ionization chamber parts in mass spectrometry will deactivate metal surfaces, preventing sample degradation, but the materials used have sufficient vapor pressure to produce organic material in the gas phase within the volume of the ionization chamber. Such material is ionized along with the sample, producing a high chemical background within the mass spectrum of the sample. Such background makes compound identification and quantitative analysis extremely difficult.
Surfaces of electropolished stainless steel have been found to give variable results, and not to be stable with time in terms of sample degradation. Hot chamber surfaces coated with gold, nickel or rhodium show degradation of samples of pesticides, such as Methoxychlor, Lindane and Parathion, of drugs and metabolites.